CONCLUSIONS

Paluxysaurus jonesi is the most complete sauropod described from the Cretaceous of North America. Paluxysaurus jonesi differs from material of the Arundel sauropod in having an expanded distal scapular blade that is not rounded on the acromial side, greater medial deflection of the proximal femur, a relatively narrower distal radius compared to mid-shaft width, and a more craniocaudally expanded proximal condyle of the tibia. The morphological differences observed between Paluxysaurus jonesi and the Arundel sauropod are too pronounced to attribute them to ontogenetic variation. Cladistic analysis based on currently available morphological evidence suggests that Pleurocoelus differs from Brachiosaurus only in presacral vertebral bone texture and robustness of the femur. The juvenile cervical vertebrae of Pleurocoelus exhibit the primitive condition for presacral bone texture. Wedel (2003) claims that such vertebrae with large pneumatic fossae would develop a polycamerate internal pneumatic structure during ontogeny but not the camellate pattern that is exhibited by Brachiosaurus and Paluxysaurus. If the latter is correct, ontogenetic variation cannot explain the different forms of vertebral pneumaticity in Pleurocoelus and other titanosauriforms, and it is highly unlikely that either Brachiosaurus or Paluxysaurus are adult forms of the Arundel taxon. Nevertheless, the phylogenetic position of Pleurocoelus is not strongly supported. If Pleurocoelus is a valid taxon, it is the most basal titanosauriform known, and would require redefinition of the taxon Titanosauriformes as the common ancestor of Pleurocoelus and the titanosaurians, and all of its descendants.

It is the opinion of the author that it has not been adequately demonstrated that the teeth of Astrodon johnstoni or those attributed to Pleurocoelus are morphologically diagnostic among titanosauriforms. Carpenter and Tidwell (2005, p. 79) note that isolated teeth cannot be referred to Astrodon with certainty, because "similar teeth occur in other taxa." Likewise, isolated teeth should not be referred to Pleurocoelus. The juvenile postcranial material that forms the type specimens of Pleurocoelus nanus and other material referred to that genus by Marsh (1888) cannot be compared to the teeth of Astrodon johnstoni. Since there is no additional associated material available for Astrodon for comparison, new discoveries should not be aligned with that genus. For the above reasoning, the argument to synonymize the two taxa, Astrodon and Pleurocoelus, seems unfounded. Furthermore, despite the fact that the morphology of some of the postcranial material from the Arundel Formation referred to Pleurocoelus by Marsh (1888), in particular appendicular skeletal elements, can be distinguished from other sauropod taxa, the grounds for referral is not morphological evidence. The type material of Pleurocoelus may not be diagnostic and there is no overlapping anatomy between the type and referred material of Pleurocoelus, making the basis for Marsh's (1888) conclusion to refer isolated bones from the Arundel Formation to that genus on size and provenance alone. Therefore, the status of either Astrodon or Pleurocoelus as valid taxonomic units is not well supported, and it is recommended that both be recognized as incertae sedis. Fragmentary sauropod remains from Texas referred to "Pleurocoelus" should be reassessed.

A diversity of at least four distinct sauropod taxa is now known for the Early Cretaceous of North America. A minimum of two sauropod taxa are present in Texas and Oklahoma, Paluxysaurus jonesi and Sauroposeidon proteles; and those are distinct from two taxa from Utah, Cedarosaurus weiskopfae and Venenosaurus dicorcei. Pending future discoveries from eastern and north-central North America, it remains unclear how the Arundel sauropod and the Cloverly sauropod fit into the current picture of Early Cretaceous sauropod diversity and biogeography. These taxa together have a widespread distribution across the continent, but otherwise each currently has a distribution that is relatively regionally isolated. Comparisons of the Early Cretaceous sauropod-producing vertebrate faunas in North America suggests, from currently available evidence, that the geographic ranges of the different sauropod taxa did not overlap, while other dinosaur taxa (some theropods and ornithischians) are common to the different faunas (e.g., Ostrom 1970, Jacobs and Winkler 1998). This is intriguing because the occurrences of all these taxa pre-date the completion of the Western Interior Seaway, which is generally assumed to have inhibited faunal exchange across North America in the Late Cretaceous. The distribution of sauropod taxa disagrees with the view that these large-bodied vertebrates had extensive geographic ranges. However, many of the sauropods compared in this study are only known from limited skeletal material, often from a single individual, and it may be likely that as additional sauropod material from the Early Cretaceous of North America is discovered and described, taxa will become known from multiple localities. Nevertheless, the description of the new taxon from Texas increases the diversity of sauropods in North America for the Early Cretaceous and provides more complete, associated material that can be compared to new discoveries from this time period.